Laminate barrier membranes and inflatable bladders formed from such membranes have been used in a variety of products for inflation or cushioning, including vehicle tires, balls, accumulators used on heavy machinery, and in footwear. It is often desirable to use polymeric materials that are thermoplastic to form the membranes because thermoplastic materials may be reclaimed and reformed into new articles, reducing waste during manufacturing operations and promoting recycling after the life of an article. While thermoplastic barrier films may be flexed to a certain extent due to their thinness, thermoplastic barrier films having only layers of barrier material generally do not have sufficient elasticity for many applications, particularly for applications in which the inflated bladder is subjected to high strains during use. In order to overcome this problem, the barrier materials have been blended or layered with elastic materials. Elastic materials, or elastomers, are able to substantially recover their original shape and size after removal of a deforming force, even when the part has undergone significant deformation.
Known bladder barrier films that are composites or laminates, however, can also present a wide variety of problems in footwear bladders and other bladders that are subjected to high strains. In particular, such bladder laminates may experience layer separation and peeling. When such forces are applied to the pressurized bladder, the deformation of the bladder will exert shear forces at the interfaces of the laminate layers. Repeated deformation will fatigue the interface, resulting in interlaminar separation. Delamination is particularly acute if a seam is present in the construction. Thus, interfacial peel adhesion strength is a highly significant property for laminates used to form pressurized bladders that can experience high strains in use.
Some previously known multi-layer bladders used tie-layers or adhesives in preparing the multi-layer laminates to achieve interlayer bond strength high enough to avoid the delamination problems. The use of such tie layers or adhesives, however, generally prevents regrinding and recycling of any waste materials created during product formation back into an usable product, making manufacturing more expensive and producing more waste. The use of adhesives also increases the cost and complexity of preparing laminates. These and other perceived shortcomings of the prior art are described in more extensive detail in Rudy, U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176, each of which are hereby expressly incorporated by reference.
Another approach has been to react together the two distinct materials to form a grafted copolymer layer or a grafted copolymer at the interface of the layers of the two different materials. Moureaux, U.S. Pat. No. 5,036,110, incorporated herein by reference, is an example of a grafted copolymer composition. Moureaux discloses a resilient membrane for a hydropnuematic accumulator that includes a film of a graft copolymer of a thermoplastic polyurethane and an ethylene vinyl alcohol copolymer.
In an alternate approach, membrane laminates have been described that eliminate adhesive tie layers by providing membranes including a first layer of a thermoplastic elastomer, such as a thermoplastic polyurethane, and a second layer including a barrier material, such as a copolymer of ethylene and vinyl alcohol, wherein hydrogen bonding occurs over a segment of the membranes between the first and second layers. Such laminates with layers of flexible materials and layers of fluid barrier materials are described, for example, in U.S. Pat. No. 6,082,025, issued Jul. 4, 2000; U.S. Pat. No. 6,013,340, issued Jan. 11, 2000; U.S. Pat. No. 5,952,065, issued Sep. 14, 1999; and U.S. Pat. No. 5,713,141, issued Feb. 3, 1998, each of which is incorporated herein by reference. While the membranes disclosed in these references provide flexible, “permanently” inflated, gas-filled shoe cushioning components that are believed to offer a significant improvement in the art, still further improvements, particularly in improved interfacial adhesion, are offered by the present invention.